I haven't really seen anyone else discuss this (surprisingly), but if terrorists wanted to attack Southern California setting forest fires would be one of the easiest and most effective methods. Even if our current blazes were set purposefully, I doubt it was done by Islamic terrorists though, because no one has come forward to claim credit.
Furthermore, it just doesn't seem like such an attack would fit the "style" of Islamofascist death-cultists. A lot of damage has been done, and the fires are a major disruption to California's economy, but there have been fairly few deaths. As such, the threat of arson just doesn't create the type of "terror" that Bin Laden and his cronies go for. Setting forest fires isn't a direct attack on the economy or on symbols of the economic system, doesn't target democracy or democratic institutions, doesn't make people afraid to leave their homes; in general, fires don't have the sort of widespread emotional impact that the terrorists long to foment.
Nevertheless, these fires will still cause property damage on the same order-of-magnitude as the 9/11 attacks, though with far fewer deaths.